Oxygen enriching device

ABSTRACT

An apparatus  150 , for generating an oxygen enhanced gas by removing nitrogen gas from air, includes a compressor  156  absorption columns  152   a  and  152   b  for removing the nitrogen gas from the pressurized air supplied from the compressor  156 , a flow rate measuring device  168  provided downstream of the absorption columns  152   a  and  152   b , ultrasonic oxygen concentration measuring means  170  provided downstream of the flow rate measuring device  168 . The ultrasonic oxygen concentration measuring means  170  includes means for generating a correction coefficient for the ratio between oxygen and argon gases contained in the oxygen enhanced gas on the basis of the flow rate of the oxygen enhanced gas measured.

TECHNICAL FIELD

[0001] The invention relates to an oxygen concentrating apparatus forgenerating an oxygen enhanced gas for a medical purpose, and inparticular to an oxygen concentrating apparatus improved to enableultrasonic measurements of the oxygen concentration and/or the flow rateof the oxygen enhanced gas.

BACKGROUND ART

[0002] It is well known that the propagation velocity of ultrasonicwaves through a sample gas is presented by a function of theconcentration and the temperature of the sample gas. The velocity ofultrasonic waves C (m/sec) propagating through a sample gas is presentedby flowing equation (1) with mean molecular weight M and the temperatureT (K).

C=(κRT/M)^(1/2)  (1)

[0003] Where;

[0004] κ: ratio of molecular specific heat at constant volume andmolecular specific heat at constant pressure

[0005] R: gas constant

[0006] Therefore measuring the velocity of ultrasonic waves C (m/sec)propagating through a sample gas and the temperature T (K) of the samplegas will provide the mean molecular weight M of the sample gas through acalculation. For example, the mean molecular weight M of a sample gascontaining an oxygen-nitrogen gas mixture of a mixture ratioP:(1−P)(0≦P≦1) will be calculated by M=M_(O2)P+M_(N2)(1−P), whereM_(O2): Molecular Weight of oxygen and M_(N2): Molecular Weight ofnitrogen. Therefore, the oxygen concentration P will be obtained througha calculation on the basis of the measurement of mean molecular weightM. When the sample gas is an oxygen-nitrogen mixture, κ=1.4 isreasonable over a wide range of the oxygen-nitrogen mixture ratio.

[0007] When the velocity of ultrasonic waves propagating through asample gas is C (m/sec), and the flow velocity of the sample gas is V(m/sec), the velocity of ultrasonic waves V₁ (m/sec) propagating in theforward direction relative to the sample gas flow is V₁=C+V, and thevelocity of ultrasonic waves V₂ (m/sec) propagating in the backwarddirection relative to the sample gas flow is V₂=C−V. Therefore, thevelocity of the sample gas flow V (m/sec) is calculated by followingequation (2).

V=(V ₁ −V ₂)/2  (2)

[0008] The flow rate (m³/sec) of the sample gas will be obtained bymultiplying this by the sectional area (m²) of the conduit through whichthe sample gas flows.

[0009] Methods and apparatuses for measuring the concentration of acertain gas or the flow velocity of a sample gas, using the aboveprinciple, on the basis of the propagation velocity or the propagationtime of ultrasonic waves through the sample gas have been developed. Forexample, Japanese Unexamined Patent Publication (Kokai) No. 6-213877describes an apparatus for measuring the concentration and the flow rateof a sample gas by measuring the propagation time of ultrasonic wavespropagating between two ultrasonic transducers opposingly disposed in aconduit through which the sample gas flows. Further, Japanese UnexaminedPatent Publications (Kokai) No. 7-209265 and No. 8-233718 describe anapparatus for measuring the concentration of a certain gas contained ina sample gas by measuring the propagation velocity or propagation timeof ultrasonic waves propagating through a control volume with areflecting type apparatus including a ultrasonic transducer and anopposingly disposed reflector. Further, U.S. Pat. No. 5,060,506describes an apparatus for measuring the concentration of atwo-component sample gas by measuring the changes in the velocity ofultrasonic waves.

[0010] Such a method and an apparatus for measuring the concentrationand the flow rate by using the propagation velocity of the ultrasonicwaves have problems. In the above-described method and apparatus, thesample gas includes only two components of oxygen gas and nitrogen gas.However, an oxygen concentrating apparatus actually outputs an oxygenenhanced gas including argon gas in addition to oxygen gas and nitrogengas. Further, the concentration of argon gas is not constant and changesdepending on the flow rate of the oxygen enhanced gas generated by theoxygen concentrating apparatus. Therefore, the conventional ultrasonicconcentration measuring device cannot measure the concentration ofoxygen gas accurately.

DISCLOSURE OF THE INVENTION

[0011] The invention is directed to solve the above-described problemsof the prior art and to provide an oxygen concentrating apparatusimproved to allow accurate ultrasonic measurements of the oxygenconcentration if the flow rate of the oxygen enhanced gas generated bythe oxygen concentrating apparatus changes.

[0012] According to the invention, there is provided an apparatus forgenerating an oxygen enhanced gas by removing nitrogen gas from air,comprising a pressurized air source, an absorption column for removingthe nitrogen gas from the pressurized air supplied from the pressurizedair source, a flow rate measuring device provided downstream of theabsorption column, ultrasonic oxygen concentration measuring meansprovided downstream of the flow rate measuring device. The ultrasonicoxygen concentration measuring means comprises means for generating acorrection coefficient for the ratio between oxygen and argon gasescontained in the oxygen enhanced gas on the basis of the flow rate ofthe oxygen enhanced gas measured by the flow rate measuring device.

[0013] According to another feature of the invention, there is providedan apparatus for generating an oxygen enhanced gas by removing nitrogengas from air, comprising a pressurized air source, an absorption columnfor removing the nitrogen gas from the pressurized air supplied from thepressurized air source, ultrasonic oxygen concentration and flow ratemeasuring means provided downstream of the flow rate setting device, andthe ultrasonic oxygen concentration and flow rate measuring meanscomprising means for generating a correction coefficient for the ratiobetween oxygen and argon gases contained in the oxygen enhanced gas onthe basis of the flow rate of the oxygen enhanced gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a block diagram of an oxygen concentrating apparatusaccording to a first embodiment of the invention.

[0015]FIG. 2 is a schematic block diagram of a ultrasonic oxygenconcentration measurement apparatus used in the oxygen concentratingapparatus of FIG. 1.

[0016]FIG. 3 is a block diagram of an oxygen concentrating apparatusaccording to a second embodiment of the invention.

[0017]FIG. 4 is a schematic block diagram of a ultrasonic oxygenconcentration and flow rate measurement apparatus used in the oxygenconcentrating apparatus of FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] Preferred embodiments of the invention will be describedhereinafter.

[0019] With reference to FIG. 1, an oxygen concentrating apparatus 150according to the invention is provided with two absorption columns 152 aand 152 b which are filled with a high performance Li-X type zeolite, acompressor 156, connected to the absorption columns 152 a and 152 bthrough a switching valve 154, for supplying compressed air to theabsorption columns 152 a and 152 b and ultrasonic oxygen concentrationmeasuring means 170 which is provided downstream of the absorptioncolumns 152 a and 152 b.

[0020] The switching valve 154 selects one of the absorption columns 152a and 152 b to fluidly connect it to the compressor 156. The air, drawnto the compressor 156 through a filter 158, is compressed by thecompressor 156 and supplied to one of the absorption columns 152 a and152 b, selected by the switching valve 154. The other of the absorptioncolumns 152 b and 152 a, which is isolated from the compressor 156 bythe switching valve 154, is opened to the atmosphere to release theabsorbed nitrogen gas for the regeneration of the absorbent.

[0021] The oxygen enhanced gas, generated by removing nitrogen gas inthe absorption columns 152 a and 152 b, is supplied to a production tank162 through check valves 160 a and 160 b. From the production tank 162,the oxygen enhanced gas is supplied to a ultrasonic oxygen concentrationmeasuring means 170 through a pressure regulating valve 164, a flow ratesetting device 166 and a flow rate measuring device 168. After theoxygen concentration is measured, the oxygen enhanced gas is supplied toa user or a patient through a filter 172 for removing particles from theoxygen enhanced gas.

[0022] With reference to FIG. 2, a preferred embodiment of a ultrasonicoxygen concentration measuring device, which provides the ultrasonicoxygen concentration measuring means, will be described below.

[0023] The ultrasonic oxygen concentration measuring device 100comprises a conduit 102 for flowing an oxygen enhanced gas or acalibration gas. The conduit 102 has a straight portion 108 andperpendicular portions 104 and 106 connected to the ends of the straightportion. A ultrasonic transducer 118 is fixedly provided at an end ofthe inside of the straight portion 108 as a ultrasonictransmission-reception device, and a reflector 122 is fixedly mounted tothe other end of the inside of the straight portion 108 to face theultrasonic transducer 118. In this embodiment, the distance between theultrasonic transducer 118 and the reflector 122 is referred to as a testlength.

[0024] A transmission-reception switch 124 is connected to theultrasonic transducer 118. The transmission-reception switch 124switches the operation mode of the ultrasonic transducer 118 between atransmission mode in which the ultrasonic transducer 118 transmitsultrasonic waves and a reception mode in which the ultrasonic transducer118 receives the ultrasonic waves. The transmission-reception switch 124is connected to a microcomputer 126 so that the switching operation oftransmission-reception switch 124 is controlled by the microcomputer126.

[0025] The perpendicular portion 104, disposed at the upstream siderelative to the flow direction of the gas through the conduit 102, hasan inlet port 104 a. An oxygen enhanced gas source 112 and a calibrationgas source 114 are connected to the inlet port 104 a through a supplyconduit 110. The oxygen enhanced gas source 112 includes the compressor156, shown in FIG. 1, and absorption columns 152 a and 152 b.

[0026] The calibration gas source 114 may include a vessel (not shown)for containing a calibration gas, the component and the component ratioof which are known, for example, a gas mixture including 20% of oxygenand 80% of nitrogen, and a pressure reducing valve (not shown) providedbetween the vessel and the supply conduit 110.

[0027] The calibration gas source 114 may also include a temperatureregulator 113, which provides means for changing the temperature of thedevice 100, in particular the conduit 102. In the example shown in FIG.1, the temperature regulator 113 includes a heating wire 113 a and anelectric power source 113 b for supplying the electric power to theheating wire 113 a.

[0028] The perpendicular portion 106, disposed at the downstream siderelative to the flow direction of the gas through the conduit 102, hasan outlet port 106 a. In the embodiment shown in FIG. 1, the filter 172is connected to the outlet port 106 a. The calibration gas used for thecalibration is exhausted to the ambient air through the filter 172 ordirectly through the outlet port 106 a.

[0029] Temperature sensors 116 and 120, for measuring the temperature ofthe oxygen enhanced gas or the calibration gas flowing through theconduit 102, are disposed preferably in the perpendicular portions 104and 106 so that they do not disturb the flow in the straight portion108. The temperature sensors 116 and 120 are connected to themicrocomputer 126. In this connection, if the changes in the temperatureof the oxygen enhanced gas is small, only one of the temperature sensors116 or 120 may be disposed.

[0030] A driver 128 for driving the ultrasonic transducer 118, areceiver 130 for A/D conversion of the signals from the ultrasonictransducer 118, a display unit 134 for indicating, for example, theoperating condition of the device 100 and the measurement results andmemory 133 including a nonvolatile memory device or a disc device forstoring the operation system for the microcomputer 126 and variousparameters are connected to the microcomputer 126

[0031] The operation of the ultrasonic oxygen concentration measuringdevice will be described below.

[0032] First, prior to the initiation of the normal measuring processfor measuring the concentration of a certain gas contained in the oxygenenhanced gas, the test length between the ultrasonictransmission-reception device 118 and the reflector 122 is calibrated,in accordance with the sequence described below, to obtain a referencelength L₀.

[0033] A gas mixture, the component and the component ratio of which areknown, for example an oxygen-nitrogen gas mixture of which mixture ratiois P:(1−P) (0≦P≦1), is supplied to the conduit 102 as the calibrationgas. At that time, the temperatures of the calibration gas are measuredby the two temperature sensors 116 and 120 and the mean value thereof isstored in the memory 132 as a reference temperature T₀ (K). Thereference temperature T₀ (K) may be any value which does not exceed theworking temperature range of the device.

[0034] During the supply of the calibration gas, pulses for generatingthe ultrasonic waves are transmitted to the driver 128 from themicrocomputer 126. A pulse voltage is supplied to the ultrasonictransducer 118 from the driver 128 through the transmission-receptionswitch 124. The ultrasonic transducer 118 generates ultrasonic wavescorresponding to the pulse voltage. The ultrasonic waves generated bythe ultrasonic transducer 118 propagate through the oxygen enhanced gasflowing through the straight portion 108 of the conduit 102 and arereflected by the reflector 122 to return to the ultrasonic transducer118. In order to enable the ultrasonic transducer 118 to receive thereturned ultrasonic waves, the transmission-reception switch 124switches the operation mode of the ultrasonic transducer from thetransmission mode to the reception mode right after the application ofthe pulse voltage to the ultrasonic transducer 118. The ultrasonictransducer 118 generates an electric signal corresponding to thereceived ultrasonic waves to the microcomputer 126 through thetransmission-reception switch 124 and the receiver 130. Themicrocomputer 126 calculates the propagation time t₀ (sec) on the basisof the time when the transmitted pulses are generated to the firsttransducer 118 and the time when the electric signal is received fromthe ultrasonic transducer 118.

[0035] In this connection, the ultrasonic propagation velocity C₀(m/sec) through the calibration gas at a temperature T₀ (K) iscalculated by equation (3) on the basis of above-described equation (1).

C ₀=((κRT ₀)/(M _(O2) P+M _(N2)(1−P)))^(1/2)  (3)

[0036] On the other hand, the relation

C ₀=2L ₀ /t ₀  (4)

[0037] gives the following equation.

L ₀=((κRT ₀)/(M _(O2) P+M _(N2)(1−P)))^(1/2) ×t ₀/2  (5)

[0038] Further, in the embodiment shown in FIG. 2, if the ultrasonicpropagation velocity through a static calibration gas or oxygen enhancedgas is C (m/sec), and the flow velocity of the sample gas from theultrasonic transducer 118 toward the reflector 122 is V (m/sec), thenthe ultrasonic propagation velocity from the ultrasonic transducer 118to the reflector 122 is C+V and the ultrasonic propagation velocity inthe direction of the ultrasonic waves reflected to the ultrasonictransducer 118 by the reflector 122 is C−V. Accordingly, the ultrasonicpropagation velocity measured by the apparatus 100 of the firstembodiment is the mean velocity of the reciprocating ultrasonic waves.Therefore, the flow velocity V of the sample gas is cancelled to allowthe ultrasonic propagation velocity C through the static sample gas.

[0039] These calculations are conducted by the microcomputer 126. Thetest length L₀ (m) thus calculated at the reference temperature T₀ isstored in the memory 132 as the reference length.

[0040] The reference length L₀ (m) between the ultrasonic transducer 118and the reflector 122 at the temperature T₀ (K) is calibrated accordingthe above method by supplying a calibration gas, the component and thecomponent ratio of which is known, to the device 100 and measuring thepropagation time t₀ (sec) of the ultrasonic waves generated by theultrasonic transducer 118. This calibration process can be automaticallycompleted by the microcomputer 126 through a simple operation, forexample one push of a button (not shown) provided on the device 100 whenthe calibration gas is supplied. Further, the process can be completedon the instant because the calculation itself is simple. Furthermore, ifthe relative position between the ultrasonic transducer 118 and thereflector 122 is changed due to the secular changes in the device 100,the device can be easily calibrated again to renew the referencetemperature and the reference length stored in the memory 132.

[0041] As described above, the oxygen enhanced gas, output from theoxygen concentrating apparatus, includes argon gas in addition to oxygenand nitrogen gases. The argon concentration is not constant and changeswith the flow rate of the oxygen enhanced gas produced by the oxygenconcentrating apparatus.

[0042] Table 1 shows the result of gas component analysis in relation tothe flow rate of the oxygen enhanced gas from the oxygen concentratingapparatus 150. The gas component analysis is conducted by gaschromatography. TABLE 1 Oxygen Argon Nitrogen Flow Rate ConcentrationConcentration Concentration (Liter/min) (%) (%) (%) 1.00 93.5 6.4 0.12.00 94.0 5.2 0.8 3.00 93.5 5.4 1.1

[0043] As shown in Table 1, the ratio between the oxygen, nitrogen andargon gases changes with the changes in the flow rate. In Table 1, themeasurements were conducted with the oxygen enhanced gas generated bythe oxygen concentrating apparatus 150. Although there are smalldifferences, the oxygen/argon ratio is substantially the same whenanother similar type oxygen concentrating apparatus is used. On theother hand, the differences in the type and amount of the absorbent andthe configuration of the absorption columns will result in the differentoxygen/argon ratio.

[0044] Next, correction coefficient generating means for generatingargon concentration correcting coefficient on the basis of the flow rateof the oxygen enhanced gas will be described below.

[0045] According to one method for correcting the argon concentration onthe basis of the changes in the flow rate, the mean molecular weight Mof equation (1) is directly described by using the abundance ratiobetween the oxygen and argon gases.

[0046] That is, on the assumption that the molecular weights of oxygen,nitrogen and argon gases are 32, 28 and 40, the mean molecular weightcan be presented by following equation (6), when the output rate fromthe oxygen concentrating apparatus is 1.00 Liter/min.

M=32P+40(6.4/93.5)P+28(1−P−(6.4/93.5)P)  (6)

[0047] Where, 100×P (%) is the oxygen concentration.

[0048] Further, the ratio of specific heat κ can be presented byfollowing equation (7) by using the ratio of specific heat of theoxygen-nitrogen mixture 1.4 and that of argon gas 1.67.

κ=1.4(1−(6.4/93.5)P)+1.67(6.4/93.5)P  (7)

[0049] Thus, measurements of the propagation velocity of ultrasonicwaves through the oxygen enhanced gas and the temperature of the gaswill provide the oxygen concentration 100×P (%) on the basis ofequations (1), (6) and (7).

[0050] In the above-described example, the flow rate of the oxygenenhanced gas is 1.00 Liter/min. In case of another value of the flowrate, (6.4/93.5) of the oxygen/argon ratio in equations (6) and (7) isreplaced with another oxygen/argon ratio corresponding to the flow rate.In this case, the oxygen/argon ratio is the argon concentrationcorrecting coefficient. Thus, the oxygen concentration can be accuratelymeasured by obtaining the argon concentration correcting coefficient byreferring a table on the basis of the oxygen enhanced gas flow rate.Further, the argon concentration correcting coefficient can be obtainedas a function of the flow rate with an approximate equation bypreviously obtaining the relation between the measured flow rate and theoxygen/argon ratio.

[0051] In order to facilitate the calculation, the following method isalso envisaged. First, the oxygen concentration is calculated byequation (2) with an assumption that the oxygen enhanced gas is composedof only oxygen and nitrogen gases. The oxygen concentration thusobtained is a value with the presence of argon gas neglected and,therefore, it is different from the actual value. However, theoxygen/argon ratio at a particular flow rate is previously known.Therefore, the oxygen concentration can be approximated by multiplyingthe previously calculated oxygen concentration by a factor. In thiscase, the factor provides the argon concentration correctingcoefficient.

[0052] For example, when the flow rate of the oxygen enhanced gas is1.00 Liter/min, 102.8 (%) of the oxygen concentration is calculated byequation (2) on the basis of the ratio of specific heat κ=1.4 and argongas neglected. However, if 93.5 (%) of the actual oxygen concentrationis previously known, (93.5/102.8) can be obtained as the argonconcentration correcting coefficient at 1.00 Liter/min of the flow rate.Therefore, when the flow rate of the oxygen enhanced gas is 1.00Liter/min, the oxygen concentration can be measured by multiplying theoxygen concentration, obtained by equation (2), by the argonconcentration correcting coefficient (93.5/102.8).

[0053] If the flow rate is not 1.00 Liter/min, the oxygen concentrationcan be accurately measured by previously obtaining the argonconcentration correcting coefficient in relation to the flow rate of theoxygen enhanced gas, and providing a table for referring the argonconcentration correcting coefficient relative to the flow rate or anapproximate equation of the argon concentration correcting coefficientin relation to the flow rate. The correction coefficient generatingmeans can be realized by storing such a table or an approximate equationand the above-described algorithm for generating the correctioncoefficient in the memory 132 and conducting the same by themicroprocessor 126.

[0054] Next, with reference to FIG. 3, a second embodiment of theinvention will be described below.

[0055] The second embodiment substantially the same as the firstembodiment, except for that the flow rate measuring device 168 and theultrasonic oxygen concentration measuring means 170 of the firstembodiment are replaced with ultrasonic oxygen concentration and flowrate measuring means 268.

[0056] With reference to FIG. 3, an oxygen concentrating apparatus 250is provided with two absorption columns 252 a and 252 b which are filledwith a high performance Li-X type zeolite, a compressor 156, connectedto the absorption columns 252 a and 252 b through a switching valve 254,for supplying compressed air to the absorption columns 252 a and 252 band ultrasonic oxygen concentration and flow rate measuring means 268which is provided downstream of the absorption columns 252 a and 252 b.

[0057] The switching valve 254 selects one of the absorption columns 252a and 252 b to fluidly connect it to the compressor 256. The air, drawnto the compressor 256 through a filter 258, is compressed by thecompressor 256 and supplied to one of the absorption columns 252 a and252 b, selected by the switching valve 254. The other of the absorptioncolumns 252 b and 252 a, which is isolated from the compressor 256 bythe switching valve 254, is opened to the atmosphere to release theabsorbed nitrogen gas for the regeneration of the absorbent.

[0058] The oxygen enhanced gas, generated by removing nitrogen gas inthe absorption columns 252 a and 252 b, is supplied to a production tank262 through check valves 260 a and 260 b. From the production tank 262,the oxygen enhanced gas is supplied to a ultrasonic oxygen concentrationand flow rate measuring means 268 through a pressure regulating valve264 and a flow rate setting device 266. After the concentration and flowrate of the oxygen gas are measured, the oxygen enhanced gas is suppliedto a user or a patient through a filter 270 for removing particles fromthe oxygen enhanced gas.

[0059] Next, with reference to FIG. 4, a preferred embodiment of aultrasonic oxygen concentration and flow rate measuring device, whichprovides the ultrasonic oxygen concentration and flow rate measuringmeans 268, will be described below.

[0060] The embodiment shown in FIG. 4 has substantially the sameconfiguration of the embodiment shown in FIG. 2, except for that thereflector 122 of the embodiment shown in FIG. 2 is replaced with asecond ultrasonic transducer 222, which provides a second ultrasonictransmission-reception device, disposed to face a first ultrasonictransducer 218, which provides a first ultrasonic transmission-receptiondevice.

[0061] A ultrasonic gas concentration and flow rate measuring device 200according to the embodiment shown in FIG. 4 includes a conduit 202 forflowing a oxygen enhanced gas or a calibration gas. The conduit 202 hasa straight portion 208 and perpendicular portions 204 and 206 connectedto the ends of the straight portion. The straight portion 208 comprises,in this embodiment, a conduit member having a circular section, thediameter of which does not changes along the longitudinal axis. A firstultrasonic transducer 218, providing a first ultrasonictransmission-reception device, is fixedly provided at the upstream endof the inside of the straight portion, and a second ultrasonictransducer 222, providing a second ultrasonic transmission-receptiondevice, is fixedly mounted to the downstream end of the inside of thestraight portion 208 to face the first ultrasonic transducer 218. Inthis embodiment, the distance between the first and second ultrasonictransducers 218 and 222 is referred to as a test length.

[0062] A transmission-reception switch 224 is connected to the first andsecond ultrasonic transducers 218 and 222. The transmission-receptionswitch 224 independently switches the operation mode of the first andsecond ultrasonic transducers 218 and 222 between a transmission mode inwhich the first and second ultrasonic transducers 218 and 222 transmitultrasonic waves and a reception mode in which the first and secondultrasonic transducers 218 and 222 receive the ultrasonic waves. Thetransmission-reception switch 224 is connected to a microcomputer 226 sothat the switching operation of transmission-reception switch 224 iscontrolled by the microcomputer 226.

[0063] The upstream side perpendicular portion 204 of the conduit 202has an inlet port 204 a. An oxygen enhanced gas source 212 and acalibration gas source 214 are connected to the inlet port 204 a througha supply conduit 210. The oxygen enhanced gas source 212 includes thecompressor 256 and the absorption columns 252 a and 252 b shown in FIG.3.

[0064] The calibration gas source 214 may include a vessel (not shown)for containing a calibration gas, the component and the component ratioof which are known, and a pressure reducing valve (not shown) providedbetween the vessel and the supply conduit 210. The calibration gassource 214 may also include a temperature regulator 213, which providesmeans for changing the temperature of the device 200, in particular theconduit 202. In the example shown in FIG. 4, the temperature regulator213 includes a heating wire 213 a and an electric power source 213 b forsupplying the electric power to the heating wire 213 a.

[0065] The downstream side perpendicular portion 206 has an outlet port206 a. The oxygen enhanced gas or the calibration gas used for theconcentration measurement or the calibration is exhausted through theoutlet port 206 a. A gas processing apparatus (not shown) mayadvantageously be disposed downstream of the outlet port 206 a if theexhausted gas is not suitable to directly exhaust to the atmosphere, asin the first embodiment.

[0066] Temperature sensors 216 and 220, for measuring the temperature ofthe oxygen enhanced gas or the calibration gas flowing through theconduit 202, are disposed preferably in the perpendicular portions 204and 206 so that they do not disturb the flow in the straight portion208. The temperature sensors 216 and 220 are connected to themicrocomputer 226. In this connection, if the changes in the temperatureof the oxygen enhanced gas is small, only one of the temperature sensors216 or 220 may be disposed.

[0067] A driver 228 for driving the first ultrasonic transducer 218, areceiver 130 for A/D conversion of the signals from the first ultrasonictransducer 218, a display unit 234 for indicating, for example, theoperating condition of the device 200 and the measurement results andmemory 233 including a nonvolatile memory device or a disc device forstoring the operation system for the microcomputer 226 and variousparameters are connected to the microcomputer 226.

[0068] The operation of the embodiment shown in FIG. 4 will be describedbelow.

[0069] First, prior to the initiation of the normal measuring processfor measuring the concentration of a certain gas contained in the oxygenenhanced gas, the test length between the first and second ultrasonictransducers 218 and 222 and the diameter D of the straight portion 208of the conduit 202 are calibrated to obtain the reference length L₀ andthe reference diameter D₀.

[0070] In the present embodiment, the calibration gas, identical to thatin the first embodiment, is supplied to the conduit 202 from thecalibration gas source 214 at a predetermined rate Q₀ by the flowregulating valve. At that time, the temperatures of the calibration gasare measured by the two temperature sensors 216 and 220 and the meanvalue thereof is stored in the memory 232 as a reference temperature T₀(K).

[0071] During the supply of the calibration gas, pulses for generatingthe ultrasonic waves are transmitted to the driver 228 from themicrocomputer 226. A pulse voltage is supplied to the first ultrasonictransducer 218 from the driver 228 through the transmission-receptionswitch 224. The first ultrasonic transducer 218 generates ultrasonicwaves corresponding to the pulse voltage. The ultrasonic waves generatedby the first ultrasonic transducer 218 propagate through the oxygenenhanced gas flowing through the straight portion 208 of the conduit 202and are received by the second ultrasonic transducer 222. The secondultrasonic transducer 222 generates an electric signal corresponding tothe received ultrasonic waves to the microcomputer 226 through thetransmission-reception switch 224 and the receiver 230. Themicrocomputer 226 calculates the forward propagation time t₁ (sec) onthe basis of the time when the transmitted pulses are generated to thedriver 228 and the time when the electric signal is received from thesecond ultrasonic transducer 222.

[0072] The transmission-reception switch 224 switches the operation modeof the first ultrasonic transducer 218 from the transmission mode to thereception mode right after the electric signal from the secondultrasonic transducer 222 is received and also switches the operationmode of the second ultrasonic transducer 222 from the reception mode tothe transmission mode. Thereafter, pulses for generating the ultrasonicwaves are transmitted to the driver 228 from the microcomputer 226. Apulse voltage is supplied to the second ultrasonic transducer 222 fromthe driver 228 through the transmission-reception switch 224. The secondultrasonic transducer 222 generates ultrasonic waves corresponding tothe pulse voltage. The ultrasonic waves are received by the firstultrasonic transducer 218. The first ultrasonic transducer 218 generatesan electric signal corresponding to the received ultrasonic waves to themicrocomputer 226 through the transmission-reception switch 224 and thereceiver 230. The microcomputer 226 calculates the backward propagationtime t₂ (sec) on the basis of the time when the transmitted pulses aregenerated to the driver 228 and the time when the electric signal isreceived from the first ultrasonic transducer 218.

[0073] By obtaining the mean value of t₁ and t₂, the affection of theflow of the calibration gas in the conduit 202 can be removed. Theultrasonic propagation time t₀ is defined by following equation (8).

t₀=(t ₁ +t ₂)/2  (8)

[0074] In this connection, the ultrasonic propagation velocity C₀(m/sec) through the gas at a temperature T₀ (K) is calculated by theabove-described equation (3).

[0075] On the other hand, the relation

C ₀ =L ₀ /t ₀  (9)

[0076] gives the following equation.

L ₀=((κRT ₀)/(M _(O2) P+M _(N2)(1−P)))^(1/2) ×t ₀  (10)

[0077] These calculations are conducted by the microcomputer 226. Thetest length L₀ (m) thus calculated at the reference temperature T₀ isstored in the memory 232 as the reference length.

[0078] Further, by using this reference length L₀, the forwardpropagation velocity V₀₁ (m/sec) and the backward propagation velocityV₀₂ (m/sec), relative to the flow direction of the calibration gas, arerepresented by V_(01=L) ₀/t₁ and V_(02=L) ₀/t₂. Therefore, the flowvelocity V₀ (m/sec) of the calibration gas in the conduit 202 isobtained by following equation (11), on the basis of above-describedequation (2).

V ₀=(V₀₁ −V ₀₂)/2  (11)

[0079] Multiplication of the flow velocity V by the sectional area (m²)of the straight portion 208, perpendicular to the axis of the straightportion 208 of the conduit 202, gives a conversion of the flow velocity(m/sec) to the flow rate (m³/sec). Thus, the reference diameter D₀ (m)at the reference temperature T₀ (K) of the straight portion 208 givesthe flowing equation.

V ₀π(D ₀/2)² =Q ₀  (12)

[0080] Therefore, the reference diameter D₀ (m) at the referencetemperature T₀ (K) can be obtained by flowing equation (13).

D ₀=2(Q ₀/(πV ₀))^(1/2)  (13)

[0081] The above calculation is conducted by the microcomputer 226, andthe reference diameter D₀ (m) thus obtained is stored in the memory 232.

[0082] According to the above method, the reference length L₀ (m)between the first and second ultrasonic transducers 218 and 222 iscalibrated at a temperature T₀ (K) by supplying a calibration gas, thecomponent and the concentration of which is known, to the device 200,and measuring the propagation times t₁ and t₂, in the forward andbackward directions relative to the flow of the calibration gas, fromthe first and second ultrasonic transducers 218 and 222. Additionally,by supplying the calibration gas to the device 200 at a predeterminedrate, the reference diameter D₀ (m) can also calibrated at the sametime.

[0083] According to the first embodiment, the oxygen concentrationcorrecting coefficient is generated on the basis of the flow rate of theoxygen enhanced gas measured by the flow rate measuring device 168. Onthe other hand, according to the second embodiment, the oxygenconcentration correcting coefficient is calculated on the basis of theflow rate of the oxygen enhanced gas measured by the ultrasonic oxygenconcentration and flow rate measuring device 200. The other functionsare same as the first embodiment.

1. An apparatus for generating an oxygen enhanced gas by removingnitrogen gas from air, comprising: a pressurized air source; anabsorption column for removing the nitrogen gas from the pressurized airsupplied from the pressurized air source; a flow rate measuring deviceprovided downstream of the absorption column; ultrasonic oxygenconcentration measuring means provided downstream of the flow ratemeasuring device; and the ultrasonic oxygen concentration measuringmeans comprises means for generating a correction coefficient for theratio between oxygen and argon gases contained in the oxygen enhancedgas on the basis of the flow rate of the oxygen enhanced gas measured bythe flow rate measuring device.
 2. An apparatus according to claim 1,wherein the ultrasonic oxygen concentration measuring means comprises: aconduit for flowing an objective gas the concentration of which ismeasured; a ultrasonic transmission-reception device secured inside ofthe conduit; a reflector secured inside of the conduit to face theultrasonic transmission-reception device; a transmission-receptionswitch for switching the operation mode of the ultrasonictransmission-reception device between a transmission mode in which theultrasonic transmission-reception device transmits ultrasonic waves anda reception mode in which the ultrasonic transmission-reception devicereceives the ultrasonic waves; a temperature sensor, disposed inside ofthe conduit, for measuring the temperature of the calibration gasflowing through the conduit; propagation time calculating means forcalculating the propagation time of ultrasonic waves through thecalibration gas within the conduit on the basis of the time when theultrasonic waves are transmitted by the ultrasonictransmission-reception device and the time when the ultrasonic wavesreflected by the reflector are received by the ultrasonictransmission-reception device; and means for generating a correctioncoefficient for the ratio between oxygen and argon gases contained inthe oxygen enhanced gas on the basis of the flow rate of the oxygenenhanced gas measured by the flow rate measuring device whereby theoxygen concentration of the oxygen enhanced gas is corrected on thebasis of the correction coefficient generated by the correctioncoefficient generating means.
 3. An apparatus according to claim 2,wherein the ultrasonic oxygen concentration measuring means comprises acalibration gas source for supplying the calibration gas the componentand the component ratio of which is known; and means for calibrating thereference length between the ultrasonic transmission-reception deviceand the reflector on the basis of the calculation results by thepropagation time calculating means when the calibration gas flowsthrough the conduit from the calibration gas source.
 4. An apparatus forgenerating an oxygen enhanced gas by removing nitrogen gas from air,comprising: a pressurized air source; an absorption column for removingthe nitrogen gas from the pressurized air supplied from the pressurizedair source; ultrasonic oxygen concentration and flow rate measuringmeans provided downstream of the flow rate measuring device; and theultrasonic oxygen concentration and flow rate measuring means comprisesmeans for generating a correction coefficient for the ratio betweenoxygen and argon gases contained in the oxygen enhanced gas on the basisof the flow rate of the oxygen enhanced gas measured by the flow ratemeasuring device.
 5. An apparatus according to claim 4, wherein theultrasonic oxygen concentration measuring means comprises: a conduit forflowing an objective gas the concentration of which is measured; a firstultrasonic transmission-reception device secured inside of the conduit;a second ultrasonic transmission-reception device secured inside of theconduit to face the first ultrasonic transmission-reception device; atransmission-reception switch for switching the operation mode of eachof the first and second ultrasonic transmission-reception devicesbetween a transmission mode in which the ultrasonictransmission-reception device transmits ultrasonic waves and a receptionmode in which the ultrasonic transmission-reception device receives theultrasonic waves; a temperature sensor, disposed inside of the conduit,for measuring the temperature of the calibration gas flowing through theconduit; and propagation time calculating means for calculating firstpropagation time of ultrasonic waves through the calibration gas withinthe conduit on the basis of the time when the ultrasonic waves aretransmitted by the first ultrasonic transmission-reception device andthe time when the ultrasonic waves are received by the second ultrasonictransmission-reception device, and for calculating second propagationtime of ultrasonic waves through the calibration gas within the conduiton the basis of the time when the ultrasonic waves are transmitted bythe second ultrasonic transmission-reception device and the time whenthe ultrasonic waves are received by the first ultrasonictransmission-reception device.
 6. An apparatus according to claim 5,wherein the ultrasonic oxygen concentration measuring means comprises acalibration gas source for supplying the calibration gas the componentand the component ratio of which is known; means for calibrating thereference length between the ultrasonic transmission-reception deviceand the reflector on the basis of the calculation results by thepropagation time calculating means when the calibration gas flowsthrough the conduit from the calibration gas source; and means forgenerating a correction coefficient for the ratio between oxygen andargon gases contained in the oxygen enhanced gas on the basis of theflow rate of the oxygen enhanced gas whereby the oxygen concentration ofthe oxygen enhanced gas is corrected on the basis of the correctioncoefficient generated by the correction coefficient generating means.